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Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 1
Fisheries and species interactions
Photo:Getty Images
Dag Ø. Hjermann Centre of Ecological and Evolutionary Synthesis (CEES)
Dept. of Biology, University of Oslo
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 2
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 3
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 4
The Barents Sea, the 1980s
• ”There has been several good year-classes for cod juveniles now. Although this has no immediate effects, it will not be long until the cod fisheries will be far better than now.”
Aftenposten, 12 September 1984
• ”Never has the Lofoten sea been as black as this year. (…) The worst season since fisheries statistics started in 1859”
Aftenposten, 18 April 1988
• What happened with the cod 1984-88? …species interactions!
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 5
Species interactions
(- -) interactions: competition (both species lose)
(+ -) interactions: predation (+ parasitism, disease)
(+ +) interactions: mutualism and symbiosis
We will here focus on predation (easiest to measure) and a bit on competition
• parasitism (disease) is probably also important for marine fish
• mutualism is important in tropical reef environments
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 6
Species interactions
Food web(shows only predation)
A
B C
D E
F
A
B C
D E
+- +
-
+ - + -+
-
F+
- +-
Interactions(only signs)
A B C D E F
A
B
C
D
E
F
Interactions(quantified)
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 7
Species interactions• Example: What is the net effect of B on C?
• Seems obvious that they are competitors for food E, and therefore B has a negative effect on C
A
B C
D E
+ -
- +-
F
- +
• To find the real effect, multiply interaction effects along all paths from B to C
1. Negative: reducing the abundance of E (competing for food)
2. Negative: increasing the abundance of predator A
3. Positive: increasing the abundance of F
• To find the real net effect, the strength of all interactions must in principle be quantified
1
2
3
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 8
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 9
A case study of interactions: the Barents Sea
Spitzbergen (Svalbard) Novaya
Zemlya
Franz Josef’s land
1.4 million km2 (4 times Norway’s land area), shared Norway/RussiaAverage depth: 230 mIce coverage: used to be 50-75% i March
0-10% in September
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 10
Norwegian Sea
North Sea
GreenlandSea
ATLANTICOCEAN
Spitsbergen
Gre
enla
nd
ICELAND
Scotland
NORWAY
RUSSIA
Jan Mayen
Faroe Islands
A case study of interactions: the Barents Sea
Barents Sea
Lofoten
North Atlantic
drift current
Coastal current
Large effect of the influx of warm, Atlantic water into the Barents Sea
Large variation from year to year and on longer time scales
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 11
Main components of the Barents Sea pelagic food web
Zooplankton
Phytoplankton (Norw. Sea - Barents Sea)
Capelin
Cod
Young herring
• Relatively low rate of primary production (phytoplankton), but huge area
• "Import" of zooplankton (Calanus finmarchicus) from the Norwegian Sea
• Effective transfer of energy(the "fatty food chain")
-> High production of fish on high trophic levels (cod)
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 12
The capelin
Main zooplankton feeder
Important food source for fish, mammals and
birds
Short-lived and unstable
Zooplankton
Phytoplankton (Norw. Sea - Barents Sea)
Capelin
1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006
02
00
40
06
00
80
01
00
0 Stock 1973-2006
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 13
Capelin, Mallotus villosus
Summer
Winter
Spawning
The species best able to take advantage of the production of the Barents Sea(migrates far north in summer)
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 14
North-East Arctic cod
Cod
Most important predator of fish
Largest remaining cod stock
Zooplankton
Phytoplankton (Norw. Sea - Barents Sea)
Capelin
1913 1921 1929 1937 1945 1953 1961 1969 1977 1985 1993
01
00
00
00
20
00
00
03
00
00
00
Stock 1913-2006
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 15
SeptemberFebruaryIce edge
Spawning area
Fødeareal
Gyting
North-east arctic cod(Gadus morhua)
• Long-lived (>15 år)
• Large variation in year-classes (cohorts)
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 16
Norwegian spring-spawning herring
Cod
Young herring
Main zooplankton feeder of the
Norwegian Sea
Uses the Barents Sea as a nursery (age 0-2)
Eats capelin larvae
Zooplankton
Phytoplankton (Norw. Sea - Barents Sea)
Capelin
1907 1916 1925 1934 1943 1952 1961 1970 1979 1988 1997
0e
+0
02
e+
05
4e
+0
56
e+
05
8e
+0
5
Stock 1907-2006
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 17
overwintering
spawning
Adults, age > 3 yr (summer)
age 1-2
Spawning area age 3
larvae
Norwegian spring-spawning herring, Clupea harengus
Age 0-2 years: Eats capelin larvae
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 18
So, what did happen with cod in the late 1980s?• 1970s and 1980-81: Low recruitment of the cod stock (a cold period in the Barents Sea)
• 1982-84: Very good recruitment of cod (warm years) → optimism
• …1983: also very good herring recruitment – for the first time since 1961
• 1985: The capelin stock collapses (>95% decrease)
• massive mortality of capelin larvae during the summers 1984-85
• capelin spawns at the age of 3-4 – and dies after spawning
• two years of no recruitment = stock collapse
• …also, the capelin fishery was closed too late (spring 1986)
• delayed the recovery of capelin
• 1985-1989: food shortage for the cod (+ harp seal, guillemots)
• Slow growth → small catches, delayed maturation
• Cannibalism: the 1984 year-class "disappeared" (eaten by 1983 yr-class?)
• High mortality of premature (age 3-6) cod
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 19
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 20
1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006
02
00
40
06
00
80
01
00
0
> 1 million tons of young herring
Reproductive success in capelin:depends (for a large part) on herring
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 21
Capelin: calculating the growth rate r
Lifetime reproductive output (R): the number of matures produced per mature
Generation length (T): average age of spawners
Reproductive rate (r): r = log(R)/T
Year t t+1 t+2 t+3 t+4
Matures spawning mat. 2-yr mat. 3-yr mat. 4-yr
Hjermann, Stenseth & Ottersen, MEPS 2004
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 22
Year
r
75 80 85 90 95
-1.0
0.0
0.5
1.0
1.5
log(matures)
r
0 1 2 3 4 5
-1.0
0.0
0.5
1.0
1.5
7374
7576
7778
798081
82
8384
85
86
87
88
89
90
9192
93
9495
• 1973-1980: no herring carrying capacity around log(N) = 4.5
1973-1980
1985-1995
85-95 73-80K (carrying capacity)
1980-1984
• 1985-1995: herring has returnedcarrying capacity around log(N) = 3
Capelin: observed (1975-) growth rate r
Hjermann, Stenseth & Ottersen, MEPS 2004
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 23
A (relatively) simple capelin model
log(N1t) = a1 + (1-b1)∙ log(Nmat
t-2) – c1∙( harvaut,t-2+harvwint,t-1)/BMt-2 – d1∙log(codt-1∙BMt-1
) – e1∙herrt-1 (1a)
log(N2t) = a2 + (1-b2)∙ log(N1
t-1) – c2∙harvaut,t-1/BMt-1 – d2∙log(codt∙BMimmat
t-1) – e2∙herrt (1b)
log(N3t) = a3 + (1-b3)∙log( N2
t-1- N2,matt-1) – c3∙harvaut,t-1/BMt-1 – d3∙log(codt
∙BMimmatt-1
) – e3∙herrt (1c)
log(N4t) = a4 + (1-b4) ∙log( N3
t-1- N3,matt-1) – c4∙harvaut,t-1/BMt-1 – d4∙ log(codt
∙BMimmatt-1
) – e4∙herrt, (1d)
• Proportion of matures(age) is a function of Nage
• All matures die after spawning• Coefficients (a, b, c, d) estimated using ordinary linear regression for each age
Densitydependence
Harvest Cod Herring
Hjermann, Ottersen & Stenseth, Proc. Nat. Acad. Sci. 2004
predation on larvae
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 24
A (relatively) simple capelin model
log(N1t) = a1 + (1-b1)∙ log(Nmat
t-2) – c1∙( harvaut,t-2+harvwint,t-1)/BMt-2 – d1∙log(codt-1∙BMt-1
) – e1∙herrt-1 (1a)
log(N2t) = a2 + (1-b2)∙ log(N1
t-1) – c2∙harvaut,t-1/BMt-1 – d2∙log(codt∙BMimmat
t-1) – e2∙herrt (1b)
log(N3t) = a3 + (1-b3)∙log( N2
t-1- N2,matt-1) – c3∙harvaut,t-1/BMt-1 – d3∙log(codt
∙BMimmatt-1
) – e3∙herrt (1c)
log(N4t) = a4 + (1-b4) ∙log( N3
t-1- N3,matt-1) – c4∙harvaut,t-1/BMt-1 – d4∙ log(codt
∙BMimmatt-1
) – e4∙herrt, (1d)
Hjermann, Ottersen & Stenseth, Proc. Nat. Acad. Sci. 2004
-0.5
0.0
0.5
1.0
1.5
2.0
Eq. 1a(age 1.5)
Eq. 1b(age 2.5)
Eq. 1c(age 3.5)
Eq. 1d(age 4.5)
Sta
ndar
dize
d re
sidu
als
**
** *
***
** * (*)
*** ***
* *
(*)
(*) (*)
a (density-dependence) b (harvest) c (cod) d (herring)
herring predation on larvae
cod predation on age 1
• Key processes: herring predation on larvae and cod predation on age 1
• More detailed analysis (in prep.) shows that harvest has no effect on reproduction (exception: 1985-86)
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 25
Capelin model, simulations
Year
Lo
g(a
bu
nd
an
ce)
1982 1984 1986 1988 1990
34
56
YearL
og
(ab
un
da
nce
)1990 1992 1994 1996 1998 2000
34
56
7
1
0.8 0.9 1.0 1.1 1.2
0.8
0.9
1.0
1.1
1.2
Year
Lo
g(a
bu
nd
an
ce)
1982 1984 1986 1988 1990
12
34
5
Year
Lo
g(a
bu
nd
an
ce)
1990 1992 1994 1996 1998 2000
23
45
6
Observed Full model Model w ithout - Cod - Herring - Herring competition - Harvest
The 1980s
All reduced modelsperform less well- cod, herring and harvest are all important
Hjermann, Ottersen, Stenseth, Proc. Nat. Acad. Sci. 2004
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 26
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 27
Herring predation on capelin indirectly affects cod cannibalism
Capelin
Cod age3-6
AbundantHerring age 1-2
Cod age 1-3
Little capelin
→ Much cod cannibalism
some years after good
herring reproduction:
Capelin
Cod age3-6
Cod age 1-3
ScarceHerring age 1-2
Much capelin
→ Little cod cannibalism
some years after bad herring
reproduction:
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 28
Ncodage 3-6, t-t
BMcapelint-t
) + f ∙Tempt-3 – g ∙
(cannibalism)
(Beverton-Holt)
log(Ncodage 3, t) = log( a*BMcodspawners, t-3
1+c*BMcodspawners, t-3
(climate effect)
A model for cod recruitment
p = 0.01p < 0.001Fitted back to 1973 (start of capelin time series):
Hjermann et al., Proc. Roy Soc. 2007
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 29
Ncodage 3-6, t-t
BMcapelint-t
) + f ∙Tempt-3 – g ∙
(cannibalism)
(Beverton-Holt)
log(Ncodage 3, t) = log( a*BMcodspawners, t-3
1+c*BMcodspawners, t-3
(climate effect)
Capelin can be replaced by herring age 1-2
F(Herring age 1-2)
9 10 11 12 13 14
56
78
9
log(HIt)
log(
Cap
elin
bio
mas
s)
BMcapelin
BM herring age 1-2(with time lags)
Now the cod model can be
fitted from 1921
(no longer limited by capelin data)
Hjermann et al., Proc. Roy Soc. 2007
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 30
Ncodage 3-6, t-t
BMcapelint-t
) + f ∙Tempt-3 – g ∙
(cannibalism)
(Beverton-Holt)
log(Ncodage 3, t) = log( a*BMcodspawners, t-3
1+c*BMcodspawners, t-3
(climate effect)
Capelin can be replaced by herring age 1-2
F(Herring age 1-2)
p = 0.003p = 0.003Fitted back to 1921:
Hjermann et al., Proc. Roy Soc. 2007
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 31
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 32
Species interactions and fisheries
• The classical Gordon-Schaefer model
• Predation not taken into account – but can be seen in the framework of this model
• But: predation is not constant
Pop. growth Fishing
Fishing + predation
Biomass added or removed
per year
Pop. growth + predation on recruiting stages
Population biomass
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 33
number of capelin (age >1 yr)
log ca
tch +
cod
(in ca
tch e
quiva
lents)
0 2000 4000 6000
750
1000
1500
2000
r < - 0 . 7 - 0 . 7 < r < - 0 . 4 - 0 . 4 < r < 0 0 < r < 0 . 4 0 . 4 0 0 . 4 0 . 4 0 . 4 0 .7 - 0 . 7 < r < - 0 . 4 0 . 4 < r < 0 . 7 r > 0 . 7
m u c h h e r r i n g m u c h h a r v e s t
1974
1996
19901985 1981
1993
Labels = year-class
Capelin: combining fishing and cod predation (i.e., predation after larval stage)
Harvest and cod predation high+ small capelin stock
= strong declineHarvest and cod predation high
+ large capelin stock= equilibrium
Consumption = productio
n
Pop. growth
(surplus)
N
Combined cod predation and
harvest *)
Hjermann, Ottersen, Stenseth, Proc. Nat. Acad. Sci. 2004
GROWTH
DECLINE
Harvest closed= increase
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 34
The time of huge capelin catches (3 mill. tons) is gone (hopefully)
Herring stock by age
1907 1916 1925 1934 1943 1952 1961 1970 1979 1988 1997
0e
+0
02
e+
05
4e
+0
56
e+
05
8e
+0
5
golden age of capelin fisheries
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 35
For the cod: harvest of cod makes the cod more sensitive to shortage of capelin
Population growth rateof cod (r)
Cod stock increasing
Cod stock decreasing
Without fishing: need 3 capelin per cod
With fishing: need 24 capelin per cod
Durant, Hjermann, Sabarros, StensethEcol. Appl. 2008
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 36
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 37
Fishing/harvesting in a multi-species food-web
• Food-webs are extremely complex (even in sub-arctic environments)
• Models must be strongly simplified versions of reality
• What do we lose by simplification?
• Approaches:
• Age-(and size?)-structured models of a few species
• Mass-balance models for a larger number of species(Ecopath with Ecosim)
• Work in the Barents Sea has concentrated on the first approach
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 38
A simplified Barents Sea ecosystem
capelin young herring
cod
zooplankton
phytoplankton
benthicinvertebrates
other pelagic prey
harp sealminke whale
man
Even this simplified version is extremely complex
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 39
A simplified Barents Sea ecosystem
capelin young herring
cod
zooplankton
phytoplankton
benthicinvertebrates
other pelagic prey
harp sealminke whale
man
Climate • Cod and herring reproduction
strongly correlated to climate
• Most complex existing age-structured model (Schweder et al. 1998)
Schweder et al.: each minke whale results in 5 tons less fish
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 40
The effects on the Barents Sea cod of reducing the capelin harvest (3 species model)
Red lines: Simulations with capelin harvest effort reduced by 75%
7 % increase
Year
0 10 20 30 40 50
-6-4
-20
24
Capelin matures biomass (log-scale)
Year
0 10 20 30 40 50
05*
10^5
1.5*
10^6
2.5*
10^6
Cod biomass age 5-10
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 41
Are models with few species the best possible approach – or are they dangerous simplifications?
• "In these models, the Barents Sea ecosystem, composed of at least 144 fish species, is reduced to four components: northern minke whales, herring, cod, and capelin" (Corkeron 2006)
• Peter Yodzis
• Need to take all possible indirect interactions into account (Yodzis TREE 2001)
• The form of the predators' functional and numerical response is important
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 42
The importance of knowing the complete food web
• Wrong decisions can be made based on mistaken beliefs of food web interactions
• Yodzis' example: Should fur seals in Southern Benguela be culled?
deep-water Cape hake
(Merluccius paradoxis)
fisheries fur seals fisheries fur seals
shallow-water Cape hake
(Merluccius capensis)
deep-water Cape hake
(Merluccius paradoxis)
Yodzis TREE 2001
YES(if maximising hake
fisheries is the goal)
? (depends on interaction strengths)
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 43
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 44
Which way is the ecosystem ”controlled”?• Bottom-up control
• populations are regulated by their food supply
• ultimately, the abundance of top predators (e.g., marine mammals and birds) are controlled by primary production (photosynthesis), which again is controlled by climate and nutrient levels
• Top-down control
• populations are regulated by their predators (including fisheries)
• changes in the abundance of key predators affects the entire food chain
• Wasp-waist control (Cury)
• Pelagic zooplankton-feeding fish are only made up by one or two extremely numerous species (e.g. anchovy, sardine)
• Changes in these key species affects the food web ”above” and ”below”
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 45
Bottom-Up Top- Down Wasp-Waist Phillippe Cury, Lynne Shannon
Which way is the ecosystem ”controlled”?
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 46
Which way is the ecosystem ”controlled”?• Or to rephrase the question: at different levels of the food web, how important is predation?
• less important than food limitation: bottom-up control
• more important than food limitation: top-down control
• for piscivorous fish, less important than food limitation; for zooplanktonn and phytoplankton, more important than food limitation: wasp-waist
• Bottom-up control traditionally thought to be dominating for marine fish
• Spatial variation between areas is clearly bottom-up controlled (i.e. fish abundance is controlled by primary productivity), e.g. Ware and Thomson (Science 2005)
• Freshwater: Top-down control found to be important
• Numerous manupulative studies showing ecosystem effects of removing top predators
• Even applied: Removal of top predators to decrease algae
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 47
Top-down effects in freshwater ecosystems
Fishing large pike improves the water quality
Pike
Zooplanktonpredators
Daphnia
Algae
–
+
–
Cannibal pike
+
–
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 48
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 49
Size-based predation can cause multiple equilibria: the lake Takvatn experiment
• Original top predator: trout. Charr introduced in the 1930s
• 1980: almost no trout + a lot of slow-growing charr
• Experimental fisheries 1984-1989: 666 000 charr removed
• Result: 30-fold increase in trout; stable 25 years after exp. fishing ended
• Explanation: exp. fishing decreased competition among charr, increased the amount of 10-15 cm charr → food for trout (and cannibalistic charr)
• The charr overcompensates for predation (survivors mature faster)
Charr
Trout
Persson et al., Science 2007
Charr length distribution
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 50
Can harvesting push marine ecosystems to alternative equilibria?
• Donald Strong: top-down control represents a form of biological instability caused by overfishing
• Ken Frank: overfishing is more likely to cause such changes in northern areas
• Lack of recovery in several Northwest Atlantic cod populations: overfishing has disrupted the predator's cultivation of its prey? (deRoos + Persson)
• In a pristine ecosystem, cod keeps its prey populations (pelagic fish) small and with high recruitment
• Overfishing: the pelagic fish gets an upper hand by competing with/predating on juvenile cod
• Closing of fisheries: the system stays in the new stable state
• Similar explanations suggested for the Baltic Sea and for the Black Sea
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 51
Multiple equilibria in the Newfoundland cod stocks
No or little recovery of several cod stocks since stock collapses in 1992 – in spite of closed fisheries
The ”cultivation-depensation” hypothesis:
Much piscivorous cod→ little forage fish
→ little competition+ predation effects on juv. cod
→ good cod recruitment
overfishingLittle piscivorous cod
→ much forage fish → juvenile cod heavily
affected by competition+ predation
→ bad cod recruitment
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 52
Structure of talk• Introduction: food-webs and species interactions
• Example: Interactions among Barents Sea fish stocks
• Cod, capelin and herring
• Effects of predation on capelin
• …which affects cod recruitment
• Species interactions and fisheries
• Combining fisheries and predation: a capelin example
• Modelling ecosystems: strategies
• Ecosystem functioning and fisheries
• Bottom-up, top-down, wasp-waist
• Alternative equilibria in freshwater and marine(?) ecosystems
• Species interactions and climate
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 53
Species interactions & climate• External forcing, i.e. climate, extremely important in boreal ecosystems
• Predation can transfer climatic effects from one species to another
• Example: The recruitment of cod and herring in the Barents Sea are closely linked to climate of the spawing year
• Capelin recruitment is not much affected by climate of the spawning year
• …but is indirectly heavily affected by climate (with 1-4 years delay) through cod and herring
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 54
How does climate influence capelin biomass?Importance - and instability - of indirect pathways
Temperature
Cod Cod x Herr.
Capelin spawning stock
Herring
1981-1983
Dire
ct
Temperature
Cod Cod x Herr.
Capelin spawning stock
Herring
1984-1987
Dire
ct
Temperature
Cod Cod x Herr.
Capelin spawning stock
Herring
1988-1990
Dire
ct
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 55
Conclusions• Species interactions are very important and cannot be overlooked
• Predation effects are more visible in northern ecosystems, but only because there are fewer species?
• Strategies for modelling ecosystems: keep models simple?
• Predation can cause the existence of multiple equilibria – overfishing can cause transitions between equilibria
Workshop on Ecosystem Based Fisheries Management – FAME, Esbjerg 27 March 2008 – slide 56
Thank you!
Photo:Getty Images
Dag Ø. Hjermann Centre of Ecological and Evolutionary Synthesis (CEES)
Institute of Biology, University of Oslo, Norway
Thanks to:
Geir Ottersen (Havforskningsinstituttet i Bergen)Nils Chr. Stenseth (CEES og Havforskningsinstituttet )
Mia Eikeset, Gjert Dingsør, Joel Durant (CEES)